The present invention relates to a method for coating micro electromechanical devices to provide coatings on such devices that are relatively corrosion-resistant and suitable for in vivo implantation, such as within a human body.
In many medical situations, it is desirable and often necessary to implant relatively small (micro) electromechanical devices for an extended period of time. For example, it may be desirable to continually administer fluid medication (either as a gas or a liquid) to a patient over an extended period of time. Examples of such treatments included the low dose continual administration of morphine for pain control, the administration of FUDR for cancer chemotherapy, the administration of baclofen for the treatment of intractable spasticity, and the like.
In such instances, a particularly desirable goal is to maintain a relatively constant level of medication in the patient""s bloodstream. In order to accomplish this goal, relatively small fluid handling devices are implanted within a patient""s body. However, both the medication and bodily fluids that may contact the micro fluid handling devices are typically corrosive. Thus, it is desirable to provide a corrosion-resistant layer to at least one surface of the micro fluid handling device to prevent or limit corrosion. For example, a nominal layer of a corrosion-resistant substance may be deposited on a substrate by sputtering by using an e-beam evaporator, where suitable corrosion-resistant substances may be silicon, gold, platinum, chrome, titanium, zirconium, and oxides of silicon or these metals. See, U.S. Pat. Nos. 5,660,728; 5,702,618; and 5,705,070 all to Saaski et al. It is described that the oxides may be formed by thermally oxidizing the corrosion-resistant substance in air after it has been applied to the substrate.
What is yet needed is a method for coating micro electromechanical devices that provides a relatively corrosion-resistant and electrically insulating coating on at least one surface of the device. Furthermore, it is highly desirable to coat the device at a relatively low temperature that will likely increase the fabrication process because substantially all of the device components and features can be assembled prior to coating the device. For example, in a typical device fabrication process, a corrosion-resistant coating is applied to individual components along the fabrication process but prior to complete assembly of the device. Because typical coating methods utilize relatively high temperatures, coating a completely assembled device is generally not possible because the relatively high coating temperatures tend to be detrimental to electrical components that, in turn, may ultimately adversely affect the functioning of the device.
As used herein, xe2x80x9ccorrosionxe2x80x9d refers to a complex electrochemical degradation of a conductive material (such as a metal or a metal alloy) or a semiconductive material (such as silicon or carbon) due to a reaction between such materials and the environment, usually an aqueous electrolyte-containing environment that can be an acidic or basic (alkaline) environment. In general, a corrosion product of such a material is in the form of an oxide of the material, such as a metal oxide, silicon dioxide, and the like. While not wishing to be bound by any particular theory, it is believed that corrosion occurs when the material (such as copper or silicon) contacts an electolytic solution and a mini-electrochemical circuit is formed when a small amount of the material dissolves in the water and combines with oxygen or other dissolved species. In forming the mini-electrochemical circuit, an imbalance of electrons between the solution and the surrounding material creates a minute flow of electrons, or current. So long as a current is allowed to flow, the material will continue to deteriorate, resulting in degradation and even pitting of the material. An electrically insulating coating is one that prevents completion of the current in the xe2x80x9cminielectrochemical curcuit.xe2x80x9d
Accordingly, one aspect of the present invention provides a method for coating an implantable device. Preferably, coating the implantable device is accomplished at a low temperature. xe2x80x9cLow temperature,xe2x80x9d as used herein, means that an input of energy to increase the temperature during plasma deposition is not required. In accordance with the present invention, plasma deposition preferably occurs at about ambient temperature, typically from about 20EC to about 30EC.
A method for coating a surface of an implantable device preferably includes plasma pretreating at least one surface of the implantable device with an inert gas; providing the implantable device to a plasma reaction chamber; and plasma treating the at least one surface of the implantable device with a reactant monomer to form a coating thereon. xe2x80x9cInertxe2x80x9d refers to relative chemical inactivity of a compound under ambient conditions however, under some plasma deposition conditions the xe2x80x9cinertxe2x80x9d compound may become reactive when a glow discharge of the compound is created.
Preferably, the reactant monomer is selected from the group consisting of a substituted or unsubstituted alkene, arene, silane, siloxane, and a combination thereof. More preferably, the reactant monomer is selected from the group consisting of ethylene, 2-methyl-1-pentene, xylene, divinylmethylsilane, hexamethyldisilane, tetramethylsiloxane, and a combination thereof.
A method in accordance with the present invention preferably includes plasma treating the at least one surface by creating a glow discharge of the reactant monomer in the presence of an inert gas. The inert gas is preferably selected from the group of argon, helium, nitrogen, neon, and a combination thereof. The reactant monomer is preferably in a ratio with the inert gas of about 3 parts to about 6 parts reactant monomer to about 1 part inert gas.
A method in accordance with the present invention preferably includes plasma treating the at least one surface by creating a glow discharge of the reactant monomer using a power of about 30 Watts to about 100 Watts for a time period from about 10 minutes or less. Preferably, the method includes a pressure within the reaction chamber of about 0.025 Torr to about 0.1 Torr.
In accordance with the present invention, plasma pretreating the at least one surface includes supplying the inert gas to the reaction chamber at a flow rate of about 2 sccm. Preferably, plasma pretreating the at least one surface includes a pressure in the reaction chamber of about 5 mTorr to about 15 mTorr. Plasma pretreating the at least one surface preferably includes generating a glow discharge of the inert gas using radio frequency (abbreviated herein xe2x80x9cR.F.xe2x80x9d) power of about 100 Watts for a time of about 2 minutes or less.
Additionally, a method in accordance with the present invention may further include cleaning the at least one surface prior to plasma treating the at least one surface with a reactant monomer. Preferably, cleaning the at least one surface is accomplished prior to plasma pretreating the at least one surface.
Also in accordance with the present invention, the method may further include adding a polymer to the at least one surface having a coating thereon, wherein the polymer is selected from the group consisting of a natural hydrogel, a synthetic hydrogel, silicone, polyurethane, polysulfone, cellulose, polyethylene, polypropylene, polyamide, polyimide, polyester, polytetrafluoroethylene, polyvinyl chloride, epoxy, phenolic, neoprene, polyisoprene, and a combination thereof. The method may also include adding a bio-active compound to the at least one surface having a coating thereon. Preferably, the bio-active compound is selected from the group consisting of an antithrombotic agent, an antiplatelet agent, an antimitotic agent, an antioxidant, an antimetabolite agent, an anti-inflammatory agent, and a combination thereof.
An implantable device coated in accordance with the present invention can be selected from the group consisting of a pacemaker, a pacemaker-cardioverter-defibrillator, an implantable neurostimulator, a muscle stimulator, an implantable monitoring device, an implantable fluid handling device, a defibrillator, a cardioverter/defibrillator, a gastric stimulator, a drug pump, and a hemodynamicmonitoring device.
Another aspect of the present invention provides an implantable device including at least one surface coating formed by the method described above. Preferably, the coating has a thickness of about 200xcex94 to about 2000xcex94. In accordance with the present invention, the at least one surface can include a metal, a nonmetal, and a combination thereof.
Yet another aspect of the present invention provides an implantable device including at least one surface having a coating formed thereon from a compound selected from the group consisting of ethylene, 2-methyl-1-pentene, xylene, divinylemthylsilane, hexamethyldisilane, and tetramethylsiloxane.
As used herein, xe2x80x9creactantxe2x80x9d monomer refers to a branched or unbranched hydrocarbon that can be plasma deposited on a substrate, preferably at a relatively low temperature. The hydrocarbon can be classified as an aliphatic monomer, a cyclic monomer, or it can include a combination of aliphatic and cyclic groups (e.g., alkaryl and aralkyl groups), wherein the hydrocarbon may include one or more heteroatoms, such as nitrogen, oxygen, sulfur, silicon, etc. In the context of the present invention, the term xe2x80x9caliphaticxe2x80x9d means a saturated or unsaturated linear or branched hydrocarbon. This term is used to encompass alkyl, alkenyl, and alkynyl compounds, for example. The term xe2x80x9calkylxe2x80x9d means a saturated linear or branched hydrocarbon, including, for example, methane, ethane, isopropane, t-butane, heptane, dodecane, and the like. The term xe2x80x9calkenylxe2x80x9d means an unsaturated linear or branched hydrocarbon with one or more carbon-carbon double bonds, such as a vinyl-containing compound. The term xe2x80x9calkynylxe2x80x9d means an unsaturated linear or branched hydrocarbon with one or more triple bonds. The term xe2x80x9ccyclicxe2x80x9d means a closed ring hydrocarbon that is classified as an alicyclic, aromatic, or heterocyclic compound. The term xe2x80x9calicyclicxe2x80x9d means a cyclic hydrocarbon having properties resembling those of aliphatic hydrocarbons. The term xe2x80x9caromaticxe2x80x9d or xe2x80x9carenexe2x80x9d compound means a mono- or polynuclear aromatic hydrocarbon.
A method in accordance with the present invention is suitable for any implantable device but is particularly well suited for micro eletromechanical devices, such as implantable pumps, filters, valves, cardiac pacesetters, and the like.